VOIP upgrade and house have around 5 headset, so what is the best suggestion

My parent house received the letter from Vodafone that his area is due for the upgrade on the VOIP...

He has a slight problem, the house currently have 5 headset, so if we going to upgrade to the VOIP, how is this going to work? (2 currently on a jetpoint that currently have 3 phone, then the other one with two extended uniden phone)